environment -- same as the &environment argument to
macro expansion functions and is used to distinguish between
compile-time and run-time environments.
The &environment argument has
dynamic extent; the consequences are undefined if
the &environment argument is
referred to outside the dynamic extent
of the macro expansion function.

When using setf of find-class, any errorp argument is evaluated
for effect, but any values it returns are ignored; the errorpparameter is permitted primarily so that the environmentparameter
can be used.

The environment might be used to distinguish between a compile-time and a
run-time environment.